The invention relates generally to active clearance control for turbomachinery and, more particularly, to rub detection for turbine engines.
In a turbine engine or other such rotating machinery, the clearance (gap) between the blades and the shroud is an important design and operational parameter of the engine. For aircraft engines, during different parts of the flight cycle, various thermal and mechanical effects lead to the gap changing. In some of these instances, that change is not uniform across the shroud or around the circumference of the engine.
Modern engines incorporate active clearance control systems to minimize these changes. Clearances are traditionally controlled using an analytic model with a built in safety margin to avoid rubs. Such models are necessarily very conservative, resulting in a less than optimal demand clearance.
In systems employing a clearance sensor, eccentricities in the clearance can result in the sensor measuring a positive clearance number, even when a rub has occurred elsewhere around the engine. Thus, for a system in which an active clearance control strategy is employed, this would result in the control driving the clearance to smaller values, even in the case where a pinch point exists and a rub has occurred. If this situation occurs, then the clearance control system runs the risk of rubbing away a substantial portion of the blade squealer tips to the point that subsequent closure of the clearance becomes impossible. However, for systems in which a clearance/displacement sensor is employed, it would be possible to do a rudimentary rub avoidance, through the use of a safety margin, in conjunction with the clearance measurement system, in order to account for some rotor or shroud eccentricities. This approach, however, lacks the desired sensitivity and precision needed for accurate control of clearances.
It would therefore be desirable to provide a clearance control method and system using clearance sensor data for rub detection. It would further be desirable for the clearance control method and system to provide enhanced sensitivity and accuracy for rub detection, so that active clearance control strategies could be beneficially employed.